eBay:
http://kleinanzeigen.ebay.de/anzeigen/s-fahrrad/k0
Open in Chrome and use 'Google Translate'. 'Fahrrad' is your keyword.
P.S. Most German people speak better English than some English people do!!!

In the UK, a car's use of the horn is meant to be limited: Highway Code rule 112 (n.b. capitalised phrases are generally indicative of the legal position):
Use only while your vehicle is moving and you need to warn other road users of your presence. Never sound your horn aggressively. You MUST NOT use your horn :
* while stationary on the road
* ...

Check with the airline you are flying with. Airlines generally have a small surcharge for taking a bike along, since people frequently travel with sporting equipment.
I looked into flying with a bike from Canada to France last year and it was only a $30 charge on the airline I was flying with (they even accepted tandems). I can't guarantee your airline will ...

I understand that there are some places in Europe where everyone driving a car honks their horn at every corner. That's probably excessive (and relatively useless).
(For the US-ians among us:) Unlike the US, in parts of Europe (such as Norway, where I've visited several times) and other parts of the world there is no established "pecking order" for one ...

Google "german traffic signs" and you'll get a number of good references for road signs, including some nice charts. For instance: http://www.gettingaroundgermany.info/zeichen.shtml . Of course, these aren't bike-specific.

Technically not in Germany but 2 kilometers from the border, there is this market:
http://www.velostation-strasbourg.org/bourse-aux-velos.html
It is run by a non-profit. A few hundred bicycles at each event, you can put your own for sale too. I bought one of my bicycles there.

If you want to be knowledgable on the level of a professional mechanic, Matthew's answer is the way to go. However, until you can find such a position, learning more about bikes will only help.
While I can't speak to resources in Germany, I can tell you that I've learned bicycle repair from three sources: from friends, asking questions at my bike shop, and ...

Not to worry! Get yourself a part time job in the local bike shop in a supporting role - maybe not sales (unless you have the product knowledge), maybe not the workshop but something in between.
Start with what you can do - punctures, wheeling bikes in and out the yard, keeping the place tidy. A couple of weeks later have a go at assembling bikes. You will ...

You might consider contacting the ADFC (Allgemeiner Deutscher Fahrrad-Club). It is an association specifically for people who cycle as a means of transportation (rather than as a sport - though of course the two are not mutually exclusive).
They are part service organization, part environmental organization. They have local groups (Kreisverband / ...

That bike is a Swabian brand from Konrad Kotter (who had until 1982 and again in 1987 the professional racing team Kotter's Racing Team) that later went over to Albuch
The Albuch Kotter logo now looks like this:

I've ordered small and large parts from bike-discount.de. 100% satisfied and cheapest price for those specific parts at that time after comparing with at least 6 other known e-shops.
Regarding cheap german bikes, I don't know about Radon (they don't seem to be well known to the rest of the EU), but I own a canyon torque for the last 3 years and a pal of ...

I have ordered components there several times and in most cases everything went fine. The last order had some flaws but as far as I could reconstruct that was more the parcel service's fault than theirs.
I cannot tell anything about their bikes from own experience but the last time I was buying a bike (some 6 or so years ago) a lo of people seemed to ...

Check out the website of the ADFC bicycle club in Leipzig http://www.adfc-leipzig.de. The "Termine" link has a list of their events including bike rides. Also the "Radtouren"link might be of interest to you. Of course everything is in the German language.

Just try chasing the threads with the normal taps (remember the left side is reverse threaded) then try the pedals. Sometimes just chasing and cleaning will be enough, and what have you got to lose if the threads are already damaged. Some pedals have a longer section of thread than others, certain pedals may work better or contact the intact threads better.
...

Perhaps you should get at least one lesson in person from an experienced rider: who can tell you what you want to know, and, correct anything you're doing wrong which you don't even know enough to ask about.
I don't think you should "cycling in real traffic" with no experience at all and based only on what you read: there's more to it, it's more complicated ...

Here is a good place to start Road Traffic Rules And here might be helpful also Learning to Drive in Europe
Not sure how it is in Germany, but in the United States. Cars and Bikes must obey the same rules of the road and believe it or not share equal rights of the road. It would make sense that the same is true in Germany, considering how it would not be ...